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by R.D. Koski and W.R. Jacobi^* (12/14)

Quick Facts…

• Fire blight is a bacterial disease that can kill branches and whole plants of many members of the rose family, including apple, pear, quince and crabapple. .
• Symptoms include dead branches, water-soaked blossoms, light brown to blackened leaves, discolored bark, black “shepherd’s crook” twigs, and dried fruits. .
• Fire blight bacteria can be spread by insects, splashing rain or contaminated pruning tools..
• Management includes resistant varieties, cultural practices, pruning and preventive chemical sprays.

Fire blight is a bacterial disease that affects certain species in the rose family (Rosaceae). It is especially destructive to apples (Malus spp.), pears (Pyrus spp.), and crabapples (Malus spp.). The disease also can occur on serviceberries (Amelanchier spp.), flowering quinces (Chaenolmeles spp.), cotoneasters (Cotoneaster spp.), hawthorns (Crataegus spp.), quinces (Cydonia spp.), pyracanthas (Pyracantha spp.), blackberries (Rubus spp.), raspberries (Rubus spp.), and mountain ashes (Sorbus spp.).

Disease incidence varies from year to year and severity is influenced by cultivar susceptibility, tree age, succulence of tissues and spring meteorological conditions. The disease is most serious when spring temperatures during pre-bloom and bloom are warmer than average. Warm rainy springs are particularly conducive to rapid spread of the pathogen, resulting in blossom blight. Blight of twig terminals can occur in late May through June during wind driven rain events. Hail and wind damage provide wounds that allow the pathogen to enter at other times. Hot summer weather generally slows or stops the disease.

Disease Cycle

Fire blight is caused by the bacterium Erwinia amylovora. The bacteria overwinter in blighted branches and at the edge of cankers (areas of bark killed by bacteria) (Figure 1). In spring, when temperatures frequently reach 65 F, the bacteria multiply rapidly.

Figure 1: Fire blight life cycle.

Masses of bacteria are forced through cracks and bark pores to the bark surface, where they form a sweet, gummy exudate called bacterial ooze. Insects such as aphids, ants, bees, beetles, and flies, are attracted to this ooze, pick up the bacteria on their bodies, and inadvertently carry the bacteria to opening blossoms. Bacterial ooze splashed by rain can also spread the pathogen.

Once in the blossom, bacteria multiply rapidly in the nectar and eventually enter the flower tissue. From the flower, the bacteria move into the branch. When the bacteria invade and kill the cambial tissue of the branch, all flowers, leaves and fruit above the girdled area die.

Infection also can take place through natural openings in leaves (stomata), branches (lenticels), pruning wounds, insect feeding and ovipositing, and hail. Droplets of bacterial ooze can form on twigs within three days after infection.

Diagnosis

Symptoms of fire blight are first seen about the time of petal fall. Infected blossoms appear water-soaked and wilt rapidly before turning dark brown; this phase of the disease is referred to as blossom blight. As the bacterial invasion progresses, leaves wilt, darken and remain attached to the tree (Figure 2); this gives the tree a fire-scorched appearance, thus the name “fire blight.”

Figure 2: Blighted leaves on ornamental apple.

Infected twigs darken and branch tips may bend over forming a “shepherd’s crook.” During wet conditions infected tissue may exude creamy bacterial ooze in droplets or fine, hair-like strands. Infected fruits also exude bacterial ooze. Rather than dropping from the tree, infected fruits gradually dry and remain attached to the branch.

Fire blight cankers on branches or stems appear as dark discolored areas that are slightly sunken, with a narrow callus ridge along the outer edge (Figure 3). The narrow callus ridge is diagnostic for differentiating fire blight cankers from fungal cankers. Under the bark associated with a canker, the inner bark turns from green to brown, but the appearance varies depending on plant variety. Droplets of bacterial ooze may appear on the canker.

Figure 3: Sunken black canker on apple branch.

Disease Management

There is no cure for this disease, so prevention is the best solution for the management of fire blight. Fire blight management methods include: planting resistant varieties, implementing cultural practices that favor growth of the plant rather than the pathogen, pruning to remove infected plant parts, and chemical sprays. Using resistant varieties is the most effective prevention method. Spraying chemicals is not recommended for homeowners because of chemical availability, potential phytoxicity and the critical timing of sprays.

Resistant varieties: Cultivars of apple, crabapple, and pear differ in their degree of susceptibility to the bacterium (Table 1) although some cultivars are less susceptible than others, no cultivar is immune to infection when the pathogen is abundant and conditions are favorable for infection. Avoid blight susceptible apple rootstocks especially when grafted to susceptible scions (Table 2). To minimize stress that may predispose the tree to other disease-causing agents, select varieties adapted to the growing area. Local weather conditions from year to year also affect the amount of fire blight found in a variety.

Cultural practices: Minimizing rapid growth and succulent tissue will reduce the risk of fire blight developing on the susceptible young, succulent tissue. Annual pruning with avoidance of major cuts will help minimize tree vigor. Similarly, limiting the amount of nitrogen fertilizer will reduce twig terminal growth. Fertilization should be based on the results of foliar and/or soil nutrient analysis and should not be applied in excess.

Pruning: Remove all blighted twigs and cankered branches. Prune twigs and branches 8 to 12 inches below the edge of visible infection. CAUTION! After each cut, surface sterilize all tools used in pruning. Dip tools in household bleach or ethyl alcohol, or use household spray disinfectants. Spreading the blight bacteria risk is lowered if pruning is delayed until mid winter. Winter pruning can also be accomplished more efficiently because pruning tools need not be disinfected between cuts if pruning is done when trees are fully dormant. To decrease the chance of new infections, promptly remove from the site and destroy all infected branches.

To remove a canker that does not extent more than 50 percent around a large stem, first make a cut through the bark down to the wood 1 to 2 inches outside the canker margin. The cut should not have any sharp angles. Next, cut and scrape away all infected bark down to the wood. Treat exposed wounds with a 70 percent alcohol solution. The whole stem should be removed if a canker extends around more than 50 percent of the stem.

During pruning, take care to avoid unnecessary wounds to the tree. When climbing trees, wear soft-soled shoes to prevent bark injuries.

Remove fire blight infected branches during summer only if the following conditions exist:

• Infections are in young, vigorous trees and the bacteria may girdle the main stem or main branches.
• Infections are in dwarfing trees on highly sensitive rootstocks, such as M.9 or M.26.
• The number of infections in older trees is limited and can easily be removed.
• It is a dry, sunny day when there is no chance of rain for 48 hours.

Chemical sprays: Chemical sprays are preventive treatments that must be applied prior to the onset of fire blight symptoms; sprays have little effect after the onset of symptoms. Expect blossom infections and plan to apply chemical sprays if: temperatures remain between 65 F and 86 F for a day or more during flower bloom, there is at least a trace of rainfall, the relative humidity remains above 60 percent for 24 hours, there is abundant succulent shoot growth, or there are fruit injuries from hail or other agents. For specific instruction on sprays and timing please use the Midwest Tree Fruit Spray Guide. The chemicals may be sold on various trade names.

Streptomycin is an antibiotic that is acceptable for use to protect trees but may be difficult to obtain. Do not use streptomycin after symptom development since it may lead to antibiotic resistance in the bacterial population.

Aluminum tris is a bactericide used prior to and during bloom.

Copper sprays are toxic to many species of bacteria. Copper sprays are best used during dormancy and prior to bud break because they may damage leaves and young fruit. Do not apply sprays within 50 days of apple harvest or within 30 days of pear harvest. Do not mix with oils or phytotoxicity issues can occur. Copper is available in several forms and sold under various trade names, including Bordeaux mixture.

Prohexadione-calcium is a plant growth regulator that reduces longitudinal shoot growth by inhibiting gibberellin biosynthesis. Prohexadione-calcium does not possess antibacterial activity but alters host biochemistry and tissues in ways that are not favorable for infection by E. amylovora. The length of time that shoot growth is inhibited depends on the application rate and tree vigor. Prohexadione-calcium is ineffective for control of the blossom blight phase of fire blight.

Table 1: Varietal susceptability to fire blight.

Rootstocks of fruit trees also differ in susceptibility to fire blight (Table 2). Cultivars are usually grafted onto a different rootstock in order to control tree height, apple cultivars on dwarfing rootstocks usually begin bearing fruit at an earlier age compared to cultivars growing on their own rootstock.

Table 2: Susceptability of Apple and Pear Rootstocks to infection by Erwinia amylovora.

Additional Information:

Swift, C.E., Hammon, R., and Larsen, H.J. 2007. Backyard Orchard: Apples and Pears. Colorado State University Fact Sheet 2.800.

Jones, A.L. and Aldwinckle, H.S. (editors). 1990. Compendium of Apple and Pear Diseases. The American Phytopathological Society, 3340 Pilot Knob Rd., St. Paul, MN 55121.

Jones, A.L. and Sutton, T. B. 1996. Diseases of Tree Fruits in the East. North Central Regional Publication No. 45 (NCR 045). Available for $10 from Michigan State University, Bulletin Office, 10-B Agriculture Hall, East Lansing, MI, 48824-1039. This publication has excellent color photos.

Bessin, R.T., McManus, P.S., Brown, G.R. and Strang, J.G. (editors). Midwest Tree Fruit Pest Management Handbook.University of Kentucky.

Lewis, D., Domoto, P.A. and Gleason, M. (editors). 2009. Midwest Tree Fruit Spray Guide.

Durham, R.E., McNiel, R.E., Hartman, J.R., Potter, D.A., and Fountain, W.M. 1999. The Flowering Crabapple. University of Kentucky Publication ID-68. This publication contains an extensive list of crabapple cultivars and cultivar resistance to common diseases, including fire blight.

Beckerman, Janna. 2006. Disease Susceptibility of Common Apple Cultivars. Purdue University, Purdue Extension Publication BP-132-W. This publication contains an extensive list of apple and edible crabapple cultivars and cultivar susceptibility to common diseases, including fire blight.

^*R.D. Koski, Colorado State University research associate; and W.R. Jacobi, professor; bioagricultural sciences and pest management. 7/98. Revised 10/09.

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